A constant current circuit providing a flow of a constant current regardless of variations in load is one of the most basic and most important circuits in a semiconductor integrated circuit.
Conventionally, constant current circuits have been formed of circuits of a current mirror type. In the constant current circuit of the current mirror type, one of two transistors having gates connected together is diode-connected, and a constant current, which is equal to a product of a constant reference current flowing through this one transistor and a capability ratio between these transistors (more specifically, a ratio of channel widths), can flow through the other transistor connected to a load circuit kept at an independent potential.
In this constant current circuit of the current mirror type, the current setting accuracy depends on whether the transistor forming the current mirror accurately has a designed current drive capability or not. In general, a drive current Id of a transistor is expressed by the following formula (1):Id=β(Vgs−Vth)2  (1)where Vgs represents a gate voltage, Vth represents a threshold voltage, and β represents a conductance. More specifically, the setting accuracy of the drive current is affected by a conductance β determined by a manufacturing process of the transistor as well as a gate voltage, i.e., a power supply voltage, and further is affected by threshold voltage Vth of the transistor.
Japanese Patent Laying-Open No. 5-191166 has disclosed a constant current circuit for allowing setting of an intended drive current without an influence by threshold voltages Vth of transistors forming a current mirror. This constant current circuit includes a first transistor having a drain connected to a gate via a resistance R, a second transistor having a gate connected to a drain of the first transistor and having the same capability ratio as the first transistor, and a current mirror circuit, of which two transistors provide a capability ratio of K:1. Since the driving is performed by the current mirror circuit, the constant current circuit disclosed in the above reference can reduce the variations in current due to manufacturing deviation, and can set the current independently of the threshold voltages of the first and second transistors.
However, the constant current circuit disclosed in Japanese Patent Laying-Open No. 5-191166 as well as other constant current circuits using current mirrors are predicated on that two transistors forming a current mirror have the same threshold voltage Vth. For example, the constant current circuit, which is disclosed in Japanese Patent Laying-Open No. 5-191166, and includes the first and second transistors forming a current mirror, is predicated on that the first and second transistors have the same threshold voltage Vth, and that the two transistors forming the current mirror circuit driving the first and second transistors have the same threshold voltage.
Thus, the setting accuracy of the drive current lowers if two transistors forming the current mirror circuit have different threshold voltages Vth1 and Vth2, and more specifically, if threshold voltage Vth1 of a reference transistor passing a reference current therethrough is different from threshold voltage Vth2 of a drive transistor passing a drive current therethrough. Further, if threshold voltage Vth2 is larger than threshold voltage Vth1, the drive transistor may be turned off even when the reference transistor is on, in which case the drive current does not flow.
In particular, thin film transistors of a polycrystalline silicon type formed on a glass substrate or a resin substrate (which may be referred to as “TFTs” or “TFT elements” hereinafter) have threshold voltages, of which variations are larger than those of the transistors formed on silicon substrates (which may be referred to as “bulk transistors” hereinafter), and the foregoing problems remarkably appear if the constant current circuit is formed of TFTs.
In recent years, TFT liquid crystal display devices have been in the mainstream of flat-panel displays. Also, electroluminescence display devices, which are formed of TFTs of a low-temperature polycrystalline silicon type and may be referred to as “EL display devices” hereinafter, have received attention in recent few years. In these TFT liquid crystal display devices and EL display devices, it is desired to form peripheral circuits, which are formed of LSIs in conventional structures, on glass substrates together with image display portions in an integral fashion. This is because sizes of the image display device can be reduced if the image display portion and the peripheral circuit can be integrally formed on the glass substrate as described above.
In these image display devices, gradation display is performed by changing a voltage applied to pixels. Thus, the liquid crystal display devices have generally employed a voltage modulation method, in which a transmittance of liquid crystal is changed by changing voltages applied to the pixels. In the EL display devices, a display brightness of an organic light-emitting diode is changed by changing a voltage applied to the pixel, and thereby changing a current supplied to an organic light-emitting diode, i.e., a current-drive type of light-emitting element provided for each pixel.
Peripheral circuits of the image display device described above include a voltage generating circuit, which generates multiple voltages (which may be referred to as “gradation voltages” hereinafter) for driving a pixel with display brightness corresponding to image data. High operation stability is required in the voltage generating circuit providing a function of gradation display. For achieving the highly stable operation, it is important that a constant current circuit included in the voltage generating circuit performs a stable operation.
Similarly to the voltage generating circuit, high operation stability is also required in a drive circuit (analog amplifier), which receives a gradation voltage generated by the voltage generating circuit, and provides a display voltage corresponding to the received gradation voltage to data lines connected to the pixels. Further, it is required in the drive circuit to provide the precise display voltage without an offset. For the stable and precise operation of the drive circuit, it is likewise important to perform the stable operation by the constant current circuit included therein.
For reducing the sizes of the device, as described above, the voltage generating circuit and the drive circuit included in the peripheral circuits may be formed together with the image display portion on the same glass substrate in the integral fashion, and the circuits may be formed of TFTs. In this structure, however, the foregoing problem remarkably occurs in the constant current circuits formed of the TFTs, and remarkably lowers manufacturing yield of the image display devices.